The present invention relates generally to photoionization detectors, to ionization chambers for use in photoionization detectors, and to methods of use of photoionization detectors.
Several photoionization detectors are described, for example, in U.S. Pat. Nos. 4,013,913; 4,398,152; 5,561,344; 6,225,633 and 6,646,444; and in German Patent DE 19535216 C1. In a typical photoionization detector (PID), a miniature gas-discharge lamp is used to produce high-energy vacuum ultraviolet (VUV) photons. In one approach, a large high-frequency voltage is applied between electrodes which are adjacent to the lamp bulb in order to induce an ionization, excitation and photoemission process in the gas which is sealed within the lamp bulb. Some of the resulting VUV photons pass through a VUV-transmissive window in the lamp to illuminate an adjacent volume within an electrically-biased ionization chamber, into which a sample of gas is introduced. Depending on the ionization potentials of the various species in the sampled gas and the maximum photon energy of the VUV radiation, photoionization of some of the gas molecules introduced into the ionization chamber can thus occur and be detected. An electrodeless (that is, having no internal electrodes), miniature PID gas discharge lamp is described, for example, in U.S. Pat. No. 5,773,833.
Typically the ionization chamber of a PID is constructed with a housing formed integrally within the PID sensor, and at least one pair of closely-spaced electrodes is positioned within the ionization chamber. The gas to be analyzed is introduced into the chamber through at least one gas inlet and leaves the chamber through at least one gas outlet. The window of the lamp is positioned so as to illuminate the sampled gas molecules with VUV photons as they move toward or within the volume between the ionization chamber electrodes. A voltage applied between these electrodes generates a high electric field across their gap, which forces the ions and electrons resulting from the photoionization process to move toward the lower or higher potential electrode, respectively. Usually an electrometer circuit is used to measure the ion current flowing to the cathode electrode. The presence of photo-ionizable molecules in the sampled gas is thereby detected. The sensitivity of a particular PID design to a variety of ionizable compounds can be determined relative to its calibrated sensitivity to a standard compound. The use of a hand-held PID device to detect trace levels of volatile organic compounds (VOCs) is one particularly important application of this technique.
It is well known that the presence of water vapor in the gas flow (as quantified by the relative humidity) can alter the sensitivity and the background signal level of a PID. Various techniques have been developed to reduce or correct for this effect. For instance, U.S. Pat. No. 4,778,998, assigned to Mine Safety Appliances Company, describes a PID in which a humidity sensor, a temperature sensor and a microcomputer (microprocessor) are used to apply a predetermined correction factor to compensate for the cross-sensitivity of the PID to the relative humidity.
As a PID lamp is operated with its window exposed to trace hydrocarbon and organo-silicone compounds in a sample of ambient air, the window surface tends to become increasingly contaminated by a surface film which is formed from the photoionization products of these air-borne compounds. This causes the effective lamp output intensity to decrease slowly with operating time. The typical maintenance procedure for PID instruments thus requires removal of the lamp and cleaning of the window manually when the sensitivity has dropped below a certain level.
Current types of PID instruments have several substantial disadvantages. For example, U.S. Pat. Nos. 5,773,833 and 6,225,633 disclose multilayer ionization chambers for a PID which are fabricated from multiple layers of machined PTFE and stainless steel, making the ionization chambers relatively difficult and expensive to manufacture. In those designs, the multilayer ionization chambers are held together by metallic pins. The metallic pins also function as electrical contacts for the ionization chamber and removably attach the sensor ionization chamber to the remainder of the instrument. Ionization chambers similar to those described in U.S. Pat. Nos. 5,773,833 and 6,225,633 are found for example in the TOXIRAE PLUS and MULTIRAE PLUS instruments available from RAE Systems, Inc. of Sunnyvale, Calif.
Furthermore, with extended operating time the electrodes within the ionization chamber become contaminated by the process described above, resulting in leakage currents and inaccurate measurements. It is quite difficult and relatively expensive to repair or restore an ionization chamber by opening it and removing this contamination. For example, as described in the Operation Manual for the TOXIRAE PLUS sensor, its sensor ionization chamber can be gently removed from the instrument for cleaning, and the ionization chamber is to be cleaned in a methanol bath (an ultrasound bath is highly recommended). After cleaning, the sensor ionization chamber can be reattached to the remainder of the instrument. Precise alignment of the sensor ionization chamber with dedicated pin contact seatings in the remainder of the instrument is required for reattachment of the TOXIRAE PLUS and MULTIRAE PLUS sensor ionization chambers.
As an alternative to manual cleaning, an enhanced concentration of ozone is purported to loosen or remove organic deposits from these surfaces to some degree. Schemes for self-cleaning the ionization chamber and the VUV lamp window, which rely on operating the VUV lamp during exposure to an oxygen-containing atmosphere in order to generate ozone, have been described. See for example U.S. Pat. No. 6,313,638. However, these self-cleaning schemes also present disadvantages, which are discussed below.
Depending on the minimum wavelength that must be transmitted, only a small number of crystalline materials, such as CaF2, BaF2, MgF2 or LiF, are usable as VUV windows for PID lamps. The transmission of these VUV window materials reduces sharply below about 140 nm. The shortest wavelength transmission is provided by LiF optical material, but the transmission of LiF is degraded over time by color-center formation (“solarization”) in the crystal due to exposure to the VUV radiation. Indeed, product specifications for a miniature LiF-window PID gas-discharge lamp which is available from RAE Systems, Inc., of Sunnyvale, Calif., indicate that the lamp is limited to an operating life of less than several hundred hours.
An alternative method is described in U.S. Pat. No. 6,255,633 for producing a self-cleaning action on the VUV lamp window and on the internal surfaces of the ionization chamber in a PID device. This requires stopping the gas flow in the ionization chamber and operating the VUV lamp to produce a higher concentration of ozone in the static sample. However, for a lamp with a LiF window this method exacerbates degradation of the LiF material due to color-center formation by the VUV radiation, and the repeated self-cleaning cycles will use up a significant fraction of its limited available operating life. This reduction of the useful operating life applies to a lesser extent to any type of VUV lamp which is self-cleaned by methods similar to that of U.S. Pat. No. 6,255,633.
For the above reasons it is therefore desirable to develop improved photoionization detectors, ionization chambers for use in photoionization detectors, and methods of use and assembly of photoionization detectors.